Design Patterns

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Types of Design Pattern

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As per the design pattern reference book Design Patterns - Elements of Reusable Object-Oriented Software , there are 23 design patterns. These patterns can be classified in three categories: Creational, Structural and behavioral patterns. We'll also discuss another category of design patterns: J2EE design patterns.

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Creational Patterns

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These design patterns provides way to create objects while hiding the creation logic, rather than instantiating objects directly using new operator. This gives program more flexibility in deciding which objects need to be created for a given use case.

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Structural Patterns

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These design patterns concern class and object composition. Concept of inheritance is used to compose interfaces and define ways to compose objects to obtain new functionalities.

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Behavioral Patterns

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These design patterns are specifically concerned with communication between objects.

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J2EE Patterns

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These design patterns are specifically concerned with the presentation tier. These patterns are identified by Sun Java Center.

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Factory Pattern

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Factory pattern is one of most used design pattern in Java. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.
In Factory pattern, we create object without exposing the creation logic to the client and refer to newly created object using a common interface.

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Abstract Factory Pattern

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Abstract Factory patterns works around a super-factory which creates other factories. This factory is also called as Factory of factories. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.
In Abstract Factory pattern an interface is responsible for creating a factory of related objects, without explicitly specifying their classes. Each generated factory can give the objects as per the Factory pattern.

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Singleton Pattern

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Singleton pattern is one of the simplest design patterns in Java. This type of design pattern comes under creational pattern as this pattern provides one of the best way to create an object.
This pattern involves a single class which is responsible to creates own object while making sure that only single object get created. This class provides a way to access its only object which can be accessed directly without need to instantiate the object of the class.

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Builder Pattern

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Builder pattern builds a complex object using simple objects and using a step by step approach. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.
A Builder class builds the final object step by step. This builder is independent of other objects.

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Prototype Pattern

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Prototype pattern refers to creating duplicate object while keeping performance in mind. This type of design pattern comes under creational pattern as this pattern provides one of the best way to create an object.
This pattern involves implementing a prototype interface which tells to create a clone of the current object. This pattern is used when creation of object directly is costly. For example, a object is to be created after a costly database operation. We can cache the object, returns its clone on next request and update the database as as and when needed thus reducing database calls.

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Adapter Pattern

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Adapter pattern works as a bridge between two incompatible interfaces. This type of design pattern comes under structural pattern as this pattern combines the capability of two independent interfaces.
This pattern involves a single class which is responsible to join functionalities of independent or incompatible interfaces. A real life example could be a case of card reader which acts as an adapter between memory card and a laptop. You plugins the memory card into card reader and card reader into the laptop so that memory card can be read via laptop.
We are demonstrating use of Adapter pattern via following example in which an audio player device can play mp3 files only and wants to use an advanced audio player capable of playing vlc and mp4 files

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Bridge Pattern

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Bridge is used where we need to decouple an abstraction from its implementation so that the two can vary independently. This type of design pattern comes under structural pattern as this pattern decouples implementation class and abstract class by providing a bridge structure between them.
This pattern involves an interface which acts as a bridge which makes the functionality of concrete classes independent from interface implementer classes. Both types of classes can be altered structurally without affecting each other.
We are demonstrating use of Bridge pattern via following example in which a circle can be drawn in different colors using same abstract class method but different bridge implementer classes.

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Filter/Criteria Pattern

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Filter pattern or Criteria pattern is a design pattern that enables developers to filter a set of objects, using different criteria, chaining them in a decoupled way through logical operations. This type of design pattern comes under structural pattern as this pattern is combining multiple criteria to obtain single criteria.

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Composite Pattern

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Composite pattern is used where we need to treat a group of objects in similar way as a single object. Composite pattern composes objects in term of a tree structure to represent part as well as whole hierarchy . This type of design pattern comes under structural pattern as this pattern creates a tree structure of group of objects.
This pattern creates a class contains group of its own objects. This class provides ways to modify its group of same objects.
We are demonstrating use of Composite pattern via following example in which show employees hierarchy of an organization.

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Decorator Pattern

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Decorator pattern allows to add new functionality an existing object without altering its structure. This type of design pattern comes under structural pattern as this pattern acts as a wrapper to existing class.
This pattern creates a decorator class which wraps the original class and provides additional functionality keeping class methods signature intact.
We are demonstrating use of Decorator pattern via following example in which we'll decorate a shape with some color without alter shape class.

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Facade Pattern

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Facade pattern hides the complexities of the system and provides an interface to the client using which the client can access the system. This type of design pattern comes under structural pattern as this pattern adds an interface to exiting system to hide its complexities.
This pattern involves a single class which provides simplified methods which are required by client and delegates calls to existing system classes methods.

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Flyweight Pattern

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Flyweight pattern is primarily used to reduce the number of objects created, to decrease memory footprint and increase performance. This type of design pattern comes under structural pattern as this pattern provides ways to decrease objects count thus improving application required objects structure.
Flyweight pattern try to reuse already existing similar kind objects by storing them and creates new object when no matching object is found. We'll demonstrate this pattern by drawing 20 circle of different locations but we'll creating only 5 objects. Only 5 colors are available so color property is used to check already existing Circle objects

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Proxy Pattern

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In Proxy pattern, a class represents functionality of another class. This type of design pattern comes under structural pattern.
In Proxy pattern, we create object having original object to interface its functionality to outer world.

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Chain of Responsibility Pattern

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As the name suggest, the chain of responsibility pattern creates a chain of receiver objects for a request. This pattern decouples sender and receiver of a request based on type of request. This pattern comes under behavioral patterns.
In this pattern, normally each receiver contains reference to another receiver. If one object cannot handle the request then it passes the same to the next receiver and so on

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Command Pattern

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Command pattern is a data driven design pattern and falls under behavioral pattern category. A request is wrapped under a object as command and passed to invoker object. Invoker object looks for the appropriate object which can handle this command and pass the command to the corresponding object and that object executes the command.

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Iterator Pattern

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Iterator pattern is very commonly used design pattern in Java and .Net programming environment. This pattern is used to get a way to access the elements of a collection object in sequential manner without any need to know its underlying representation.
Iterator pattern falls under behavioral pattern category.

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Mediator Pattern

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Mediator pattern is used to reduce communication complexity between multiple objects or classes. This pattern provides a mediator class which normally handles all the communications between different classes and supports easy maintainability of the code by loose coupling. Mediator pattern falls under behavioral pattern category.